Thermal sterilization of foods is quantified as a time-temperature relationship. A technique was needed to measure temperatures in solid food particles in a liquid carrier fluid to determine the microbial safety of the project. Highlights: A pilot scale aseptic process was used to heat and pump potato soup; product flowed through the heat exchangers and then into a tube which passed through the bore of a horizontal 4.7T magnet. Phase mapping was used to create temperature images of potato cubes at temperatures up to 130xC. These and subsequent results may contribute to the first successful filing of a multiphase aseptic process with the US FDA.